Manufacture of fatty acid esters of sorbitan as surfactants

ABSTRACT

Fatty acid sorbitan esters by direct reaction of sorbitol with fatty acid using a phosphorus oxyacid, including reducing phosphorus oxyacid particulary phosphouous acid, catalyst system and an alkali or alkali earth metal base, particulaly oxide, hydroxide or carbonate, in an axid:base molar ratio of 0.9:1 to 1.7:1, at a calalust concentration of 1.5 to 30%, particularly 3 to 12%, by weight of sorbitol can have lower color than products of prior commercial bulk processes even without the use of activated carbon and product bleaching typically used previously. The esters can be further reacted with alkylene oxide to make desivatives, particularly polysorbates. The esters and polyalkyleneoxy detivatives are useful surfactants in various application. Products of low color made without using activated carbon or bleaching are attractive for personal care applications.

This application is a 371 PCT/GB97/02047 filed Jul. 30, 1997.

This invention relates to an improved method of making surfactantesters, especially sorbitan esters of fatty acids, to the use of theproduct esters as surfactants, and to the manufacture of alkoxylated,especially ethoxylated, surfactant esters, in particular the ethoxylatedsorbitan fatty acid esters known as polysorbates and to the use of theproduct alkoxylated esters as surfactants.

Sorbitan esters of fatty acids, such as those sold by various ICIcompanies under the Trade Mark “Span” are widely used as surfactants andas intermediates in the manufacture of relatively more hydrophilicsurfactants by alkoxylation, especially ethoxylation to make so-calledpolysorbate surfactants e.g. as sold by various ICI companies under theTrade Mark “Tween” Typically, sorbitan fatty acid esters arecommercially manufactured of a large scale by reacting sorbitol and thefatty acid in the presence of a catalyst system which promotes theesterification reaction and which also catalyses the internaletherification of the sorbitol to sorbitan. Generally the etherifcationreaction is desired only to progress to the mono-cyclic product althougha second internal etherification reaction is possible to form theiso-sorbide moiety. It is believed that the internal etherificationtakes place after the esterification reaction, but this is not directlyimportant for most large scale manufacturing methods as the reactionsare, in practice, carried out batchwise under a single stage or “onepot” protocol. As there are various sites for esterification andinternal etherification, the product is usually a mixture of isomers.Further scope for variability in the molecule is provided by thepossibility of multiple esterification. The variability of the moleculespossible is well known among those who manufacture and use thesesurfactants.

Esterification is, in principle subject to both general acid and basecatalysis and etherification is typically catalysed by acids. Typically,in the manufacture of sorbitan fatty acids esters, the catalyst systemsused are a mixture of acidic and basic catalysts. Conventionallyexplained, the base is used to catalyse the esterification and the acidto catalyse the etherification. With water being present in the system,either from supply of starting materials as aqueous solutions or waterformed during the reactions, as expected, the acid and base tend toreact to form salts. This may imply that the true catalyst is a salt orcombination or acid or base and salt. Typically the reaction temperatureis about 240° C., the catalysts are chosen so that they are bothchemically stable and non-volatile at the reaction temperatures. Usuallyconventional catalyst systems use NaOH as the base and a phosphorusoxyacid as the acid. Various phosphorus oxyacids can be usedsuccessfully as acid components of the catalyst system, but usuallynon-condensed phosphorus oxyacids such as phosphoric acid have beenpreferred historically. Conventionally, the base and acid catalystcomponents (for a typical NaOH/phosphoric acid system) are used at aweight ratio of about 1:1 corresponding to a molar ratio of about 1.3:1and at an overall level of between 0.6 and 0.8% by weight of thecombined acid and sorbitol reagents equivalent to between about 2.3 andabout 3% by weight of the sorbitol reagent.

At the elevated reaction temperatures typically used in the reaction,care needs to be taken to avoid excessive oxidation of the reagents andusually the reaction vessel is blanketed with nitrogen. Despite thissome oxidation and/or pyrolysis (possibly oxidative pyrolysis) doesusually take place and efforts have been made to reduce the extentand/or effect of these undesired side reactions on the properties of theproduct. The most obvious effect on the product is that it is typicallycoloured. Improvements in the process to reduce or remove the colouredside products include the inclusion in the reaction of carbon(“activated carbon”) to absorb coloured side products and the use ofreducing varieties of phosphorus acids, particularly phosphorous and/orhypophosphorous acids, to make the reaction environment less oxidising(possibly by the reducing acid acting as a sacrificial anti-oxidant).Often after separation of the activated carbon from the reaction productthe product is further decolourised by bleaching. Even using suchimprovements, the colour of the usually liquid product (as the neatmaterial) is typically about 8 Gardner units having a dark brown colour.In the absence of such process improvements the colour would probably bemore than 10 Gardner units. Gardener units are based on visualcomparisons and in this context probably represent an approximatelylogarithmic scale of concentration of the coloured side products.

It is known to make very pure sorbitan fatty acid esters by usingspecially purified starting materials and separating the etherificationand esterification reactions for example as is described in JP 62-142141A. However, such methods are of little use in the bulk manufacture ofsorbitan fatty acid esters as the multiplicity of purification andreaction stages makes them very expensive.

Polyalkoxylated sorbitan fatty acid ester surfactants, particularly ofthe polysorbate type, are typically manufactured by reacting thecorresponding sorbitan esters with alkylene oxide, usually ethyleneoxide, typically under alkali catalysis.

The present invention is based on the discovery that the use of acatalyst system in which the relative proportion of acid is greater thanthat used conventionally can yield sorbitan fatty acid ester productswhich have significantly improved purity, particularly improved colour(lower Gardner colour) and odour even when no activated carbon isincluded in the reaction system. Further, using such modified catalystsystems enables a higher level of catalyst to be used giving shorterreaction times, lower reaction temperatures or a combination of both,which can yield further improvements in the properties of the product.The fatty acid esters can be alkoxylated, and in particularlyethoxylated to give polysorbate type products, also showing improvedcolour and odour as compared with otherwise similar products made withconventionally made sorbitan fatty acid esters.

Accordingly, the present invention provides a method of making fattyacid esters of sorbitan which comprises reacting the fatty acid directlywith sorbitol in the presence of a catalyst system which comprises aphosphorus oxyacid, including a reducing phosphorus oxyacid, and analkali or alkali earth metal strong base in a molar ratio of acid tobase of from 0.9:1 to 1.7:1 and at a catalyst system concentration offrom about 1.5 to about 30% by weight of the sorbitol.

The invention further enables the manufacture of alkoxylated esters ofsorbitan, in particular polysorbate materials, having improvedproperties and the invention accordingly includes the use of fatty acidesters of sorbitan made by the method of the invention in themanufacture of corresponding alkoxylated esters of sorbitan, inparticular polysorbate materials, by alkoxylating and in particularethoxylating the fatty acid esters of sorbitan made according to theinvention. Specifically, the invention includes a method of makingalkoxylated esters of sorbitan, in particular polysorbate materials,comprising reacting a fatty acid directly with sorbitol in the presenceof a catalyst system which comprises a phosphorus oxyacid, including areducing phosphorus oxyacid, and an alkali or alkali earth metal strongbase in a molar ratio of acid to base of from 0.9:1 to 1.7:1 and at acatalyst system concentration of from about 1.5 to about 30% by weightof the sorbitol to form a fatty acid ester of sorbitol; and subsequentlyalkoxylating, and in particular ethoxylating, the fatty acid ester ofsorbitol by reacting the ester with an alkylene oxide, particularlyethylene oxide.

Molar ratios of acid and base refer to the ratios of the nominal H⁺ andOH⁻ content of the compounds concerned (and are thus in effectequivalent ratios of the respective acids and bases).

These ratios for phosphorus oxyacids take account of the multiplepossible protons available so that e.g. phosphorus acid is treated as adibasic acid.

The catalyst system used in the method of making fatty acid esters ofthe invention is a combination of an alkali or alkali earth metal strongbase and an acid. The base is a strong base and will usually be analkali or alkali earth metal oxide, hydroxide or carbonate, desirably analkali metal hydroxide, particularly sodium and/or potassium hydroxide.The acid part of the catalyst system includes a phosphorus oxyacid.Desirably, as typical reaction temperatures are elevated, the acidcatalyst is not volatile at reaction temperature and typically the acidpart of the catalyst system will be wholly of phosphorus oxyacids. Thephosphorus oxyacid part of the catalyst includes at least some reducingphosphorus oxyacid(s) i.e. a phosphorus oxyacid that acts as a reducingagent under the esterification reaction conditions. Desirably thereducing phosphorus oxyacid includes hypophosphorous acid and/or, andespecially, phosphorous acid. We have found that phosphorous acid ismuch more effective than hypophosphorous acid, although the reason forthis is not clear. The whole of the phosphorus oxyacid desirably isreducing acid, especially phosphorous acid, but it may be a combinationof a reducing phosphorus oxyacid and one or more non reducing phosphorusoxyacid(s) particularly phosphoric acid. If such a combination is usedthen desirably the proportion of reduced phosphorus oxyacid, especiallyphosphorous acid, is at least 5%, but usually at least 25%, particularlyat least 50, and typically up to 95% of the total phosphorus oxyacid.

The use of alkali metal hydroxide and phosphorous acid in the catalystsystem forms a specific feature of the invention which accordinglyincludes a method of making fatty acid esters of sorbitan whichcomprises reacting the fatty acid directly with sorbitol in the presenceof a catalyst system which comprises phosphorous acid and an alkalimetal hydroxide in a molar ratio of phosphorous acid to alkali metalhydroxide of from 0.9:1 to 1.7:1 and at a catalyst system concentrationof from about 1.5 to about 30% by weight of the sorbitol.

The molar ratio of acid: base in the catalyst system used in makingfatty acid esters according to this invention is in the range 0.9:1 to1.7:1, more usually 1:1 to 1.5:1, desirably 1.1:1 to 1.3:1 andparticularly about 1.2:1. In addition to an improvement of the colour ofthe fatty acid ester product from the use of the particular ratios ofacid to base according to the invention, we have found that thiscatalyst system can be a more active catalyst, speeding the reactioncompared with conventional catalyst systems. The reaction to make fattyacid esters can be yet further accelerated by using higher levels ofcatalyst than are conventional without causing more coloration of theproduct. We have obtained particularly good results using up to about 6,particularly up to about 5 times and especially up to about 3 times theamount (typically about 2.3% by weight) of catalyst based on thesorbitol that is conventional. Thus in this invention the amount ofcatalyst used is from about 1.5 to about 30%, particularly from about 3to about 12% and especially about 3 to about 8% by weight of thecatalyst system based on the sorbitol. The catalyst concentrations areexpressed based on the weight of sorbitol because this avoids apparentdiscrepancies arising from the differing molecular weights whendifferent fatty acids are used and compensates somewhat for therelatively lower amounts of catalyst (based on the reaction mixture as awhole) typically used in making higher sorbitan esters e.g. sorbitantri-fatty acid esters.

The discoloration of sorbitan fatty acid esters during manufacture is afunction of the susceptibility of the fatty acid used to oxidationduring the esterification/etherification process. Thus, it is well knownthat commercial grades of sorbitan mono-oleate tends to be more darklycoloured than the corresponding grades of sorbitan mono-stearate andthis seems to flow from the unsaturation of oleic acid. The invention isparticularly applicable to making esters of unsaturated fatty acids, butcan be used with advantage in making saturated fatty acid estersalthough the relative improvement in colour is likely to be less thanwith unsaturated acids such as oleic acid. Typical fatty acids that canbe used in the method of this invention include unsaturated fatty acidssuch as: oleic, linoleic, linolenic and erucic acids, and saturatedacids such as lauric, myristic, palmitic stearic and behenic acids. Suchfatty acids are commonly available as mixtures of fatty acids of similarcarbon chain length which are as found in the natural source from whichthey are obtained (or as mimicked by synthetic analogues), for examplecoconut fatty acids (COFA)—mainly a mixture of C₁₂ and C₁₄ acids, palmoil fatty acids—mainly palmitic acid and hydrogenated tallow fattyacids—mainly stearic acid. Such mixtures can readily be used as thefatty acid source in the method of this invention.

The grade of sorbitol used can also affect the colour of the fatty acidester product. The use of a grade with low content of reducing i.e.aldehyde or ketone containing, sugars is desirable as the carbonylgroups are recognised as likely to be relatively easily converted tocoloured products on pyrolysis, especially oxidative pyrolysis. However,the method of making fatty acid esters according to this invention cangive substantial benefits even with grades of sorbitol that are notespecially low in reducing sugars. In the method of the invention, thecolour of the product can be improved modestly by the inclusion ofmetabisulphite e.g. as sodium metabisulphite added as a solid or as anaqueous solution, in the reaction mixture. We believe that theimprovement arising from the inclusion of metabisulphite arises from theformation of a metabisulphite adduct with the aldehyde or ketone groupsof reducing sugars thus reducing the susceptibility of the systemtowards colour formation during the reaction to make the fatty acidester. The amount of metabisulphite used will typically be from 0.1 to10% by weight of the sorbitol, the amount generally corresponding to thelevel of reducing sugars in the sorbitol. This addition can give abenefit of about 0.5 to 1 Gardner unit of colour in the product fattyacid ester.

The intended fatty acid ester product can be a mono-, or higher ester asthere are nominally four free hydroxyl groups in sorbitan. Typicallymono-, sesqui-, di- and tri-fatty acid esters of sorbitan are madecommercially and similar product can be made by the method of thisinvention. In practice the products are made to meet a performancespecification as they are commercial materials and although they areoften named using terms suggesting relatively precise compounds, theproducts will often have non-integral ratios of sorbitan and fatty acidresidues. For example, commonly the product sold as sorbitan mono-oleatewill contain on average from 1.4 to 1.5 oleic acid residues per sorbitanresidue. With this in mind, for the lower esters the fatty acid andsorbitol will typically be used in approximately equimolar proportionsand the reaction will proceed substantially to completion. Where higheresters are desired, some of the fatty acid may not react with thesorbitan and will remain as (nominally) free acid in the synthesisproduct. Thus, nominal sorbitan tri-oleate typically contains about 10%unreacted oleic acid.

The method of this invention can produce fatty acid ester products,without the use of activated carbon, with a colour superior to thatobtained by otherwise similar prior art processes including the use ofactivated carbon. The use of activated carbon is not excluded in thisinvention, but its inclusion does not appear to give any significantfurther benefit. Indeed avoiding the use of activated carbon may beadvantageous as it is difficult or messy to filter from the fatty acidester reaction product and tends to retain some of the product,typically amounting to a few percent of the total yield, in the filterin a form that is not readily separable from the carbon.

Similarly, in typical prior art processes, to obtain product with a(then relatively) low colour, the fatty acid ester product wouldtypically be bleached e.g. with hydrogen peroxide. In this invention,products with good colour can be obtained without bleaching. Evenfurther improved colour can be obtained by bleaching the product of thisinvention. However, particularly for personal care applications, it canbe desirable to use non-bleached fatty acid ester products as thisobviates any risk of including bleach residues or side products frombleaching in the final products.

The reaction to make the fatty acid esters is typically carried out inan inert atmosphere, usually under a nitrogen blanket, to minimiseoxidative degradation of the starting materials or products, and at atemperature sufficiently high to drive off water present in the startingmaterials or generated by the etherification and esterificationreactions. Typically, the reaction mixture is heated to the maximumintended reaction temperature after mixing of the reagents and additionof the catalyst. Conventional maximum reaction temperatures aretypically about 240 to 250° C., but we have found that lower reactiontemperatures can be used. Thus, in this invention the peak reactiontemperature will typically be in the range 150 to 250° C. but moreusually from 170 to 230° C. The use of reaction temperatures lower thanthose that are conventional is particularly appropriate where increasedconcentrations of catalyst are used. At catalyst levels 2 to 3 timeconventional levels, the reaction temperature can be in the range 200 to230° C. and by using higher levels of catalyst e.g. up to about 6 timesthe conventional level the reaction temperature can be reduced to about170° C. if desired. The reduction in reaction temperatures seems toprovide a further benefit in the colour and purity of the product. Evenwith relatively low reaction temperatures, the reaction times using themethod of this invention can be shorter than is conventional. We haveobtained satisfactory conversion in a reaction time of 5 hours at a peakreaction temperature of 220° C. as compared with a reaction time of 8hours with a peak reaction temperature of 245° C. using a more nearlyconventional type of catalyst system (ca. 1.3:1 molar sodium hydroxide:phosphorous acid at 0.7% by weight).

The lower colour fatty acid ester products which can be made by themethod of this invention makes them particularly suitable for inclusionas dispersants and/or emulsifiers in personal care products. Specificend uses are generally associated with particular esters so thatsorbitan palmitate, stearate and behenate are particularly useful inoil-in-water creams, milks and lotions with a wide range of end useapplications; the iso-stearate and oleate in water-in-oil creams, milksand lotions and bath and massage oils, water washable ointments and indecorative cosmetics, particularly lipsticks, blushers and other make upitems, especially as pigment dispersants; and laurate in mudpacks,particularly as dispersants, and in baby shampoos, particularly asconditioners.

In addition to the advantage of lower colour, the sorbitan fatty acidesters made by the method of this invention have less odour, and usuallya less objectionable odour, than conventional materials. Thus, the odouris typically more akin to that of toffee than the burnt or rancid odoursassociated with conventional sorbitan esters as currently commerciallyavailable. A further advantage, of particular relevance to their use inpersonal care products, is that esters made by the method of thisinvention need not and generally will not include residues of bleachingmaterials because bleaching materials are not used (because as describedabove they are not necessary). This may make these materialsparticularly attractive for personal care products, such as cosmetics,that are used for long periods in contact with skin.

The invention accordingly includes personal care products, particularlyof the types mentioned above, including one or more fatty acid estercompound(s) made by the method of the invention as a dispersants and/oremulsifiers and the use of fatty acid ester compounds made by the methodof the invention as a dispersants andlor emulsifiers in personal careproducts.

The improvement in colour and odour also makes it possible to makealkoxylated products, particularly ethoxylated products of thepolysorbate type, of improved colour and odour and as noted above theinvention includes the manufacture of alkoxylated, particularlyethoxylated, sorbitan fatty acid esters and the use of sorbitan fattyacid esters made by the method of the invention in the manufacture ofalkoxylated, particularly ethoxylated, sorbitan fatty acid esters(polysorbates). The alkoxylation reaction on the sorbitan ester istypically carried out at superambient temperatures e.g. from about 125to 175° C., typically using a basic catalyst, usually an alkali metalhydroxide such as usually sodium or potassium hydroxide or alkali metalfatty acid salts. The reaction is continued until the desired degree ofalkoxylation, usually expressed as a OH number (mg KOH equivalent pergram of product), is reached. At the end of the reaction the basiccatalyst is neutralised to give an unbleached product. The improvedcolour of the sorbitan fatty acid esters may make it possible to omitthe normal post-alkoxylation bleaching step in the manufacture of suchalkoxylated products and this is a particularly advantage for personalcare products where bleaching residues are required to be minimised (andare desirably absent). The improvement in odour is also relevant forpersonal care and food additive uses of such materials. If a producthaving even lower colour is desired then the alkoxylated material may bebleached conventionally e.g. with hydrogen peroxide.

Such alkoxylated, particularly ethoxylated derivatives of sorbitanesters are used as emulsifiers and dispersants in oil-in-water emulsionsand creams and in particular as solublisers for perfumes and flavouringmaterials in personal care and food products.

The invention accordingly further includes personal care products andfood product andlor additives, particularly of the types mentionedabove, including one or more alkoxylated, particularly ethoxylated,sorbitan fatty acid ester(s) made by the method of the invention asdispersants and/or emulsifiers and/or solublisers; and the use ofalkoxylated, particularly ethoxylated, sorbitan fatty acid ester(s) madeby the method of the invention as a dispersants and/or emulsifiersand/or solublisers in personal care and/or food and/or food additiveproducts.

The improved colour sorbitan esters and aikoxylated, especiallyethoxylated sorbitan esters, made according to this invention may besusceptible to an increase in colour on storage, particularly if specialcare is not taken. To make the products less likely to deteriorate forthis reason it may be desirable to include a small proportion e.g. 0.01to 0.25% by weight of an antioxidant such as2,6-di-tert-butyl4-methylphenol in the ester products.

The following Examples illustrate the invention. All parts andpercentages are by weight unless otherwise specified.

Materials Used oleic acid Priolene 6900 ex Unichema lauric acid nominallauric acid was COFA - fatty acids derived from coconut oil (a mixtureof C12 to C14 mainly saturated fatty acids) sorbitol Sorbidex 130 exCerestar Dicalite a diatomaceous earth filter aid ex Redland MineralsTest Methods colour was measured using a Gardner Colorimeter and theresults are expressed in Gardner units (GU). acid number was measured bythe method of ASTM D974-92 and the results are expressed in mg(KOHequivalent).g(sample)⁻¹. hydroxyl number was measured by the method ofASTM E326-85 and the results are expressed in mg(KOHequivalent).g(sample)⁻¹. saponification no was measured by the method ofCAPAR4/1 and the results are expressed in mg(KOHequivalent).g(sample)⁻¹.

EXAMPLE 1

The laboratory scale esterification reactor used was a 11 flat flangedglass flask fitted with a nitrogen supply, thermometer (thermocouple), amechanical p.t.f.e. stirrer a Vigreaux column having a side armcondenser leading to a collection flask and an external isomantle. Oleicacid (416 g; 1.47 mol), sorbitol (184 g; 1 mol; as a 70% aqueoussolution) and catalyst (4.8 g; 2.6% by weight based on the sorbitol of amixture of NaOH and phosphorous acid in a molar ratio of acid:base of1.2:1) were charged to the flask, the mix was thoroughly sparged withnitrogen (and throughout the reaction) and the temperature increasedsteadily to 110° C. when water (from the sorbitol solution) started todistil from the reaction mixture. The temperature was increased slowlyto 130° C. until the removal of free water was nearly complete and wasthen increased to 245° C. over 30 minutes. The reaction was monitored byperiodic sampling and analysis for acid number until this fell below 10and then by hydroxyl number until this dropped within the range 210 to185. The amount of water distilled from the reaction was also used as anindication of the extent or the reaction. The reaction mix was thenfiltered through a medium flow filter paper using 1% by weight (based onthe weight of the reaction mixture) Dicalite as filter aid. This Examplewas repeated except that the catalyst used was 2.3% by weight of amixture of NaOH and phosphorous acid in a molar ratio of acid:base of0.8:1 as comparative Example 1C. The properties of these products wereas follows:

property Ex 1 Ex 1C Units acid number 3.3 24 mg(KOH).g⁻¹ hydroxyl number199 193 mg(KOH).g⁻¹ saponification no 154 152 mg(KOH).g⁻¹ colour 3.5 7Gardner units

EXAMPLES 2 TO 9

Example 1 was repeated using various molar ratios of acid to base andvarying amounts of catalyst. The ratios and amounts and the effect onthe colour of the product is set out in Table 1 below which includesdata from further comparative examples 2C to 4C. In comparative Example4C the acid used was phosphoric acid i.e. no reducing phosphorus oxyacidwas used.

EXAMPLE 10

Example 1 was repeated except that the molar ratio of oleic acid tosorbitan used was about 3:1 to make (nominal) sorbitan trioleate andthat the amount of catalyst used was 5.7% by weight of the sorbitol atan acid to base molar ratio of 1.2:1. Comparative Example 10C is similarto Example 10 but used a molar ratio of acid to base of 0.8:1 at a levelof about 2.8%. The results are included in Table 1 below.

EXAMPLE 11

Example 1 was repeated except that the oleic acid used in Example 1 wasreplaced with lauric acid (COFA) and that the amount of catalyst usedwas 3.2% by weight at an acid to base molar ratio of 1.2:1. ComparativeExample 11C is similar to Example 11 but used a molar ratio of acid tobase of 0.8:1 at a catalyst level of 1.6%. The results are included inTable 1 below.

EXAMPLE 12

Example 5 was repeated except that activated carbon (6.7 g; 1.8% byweight) was included in the the product was 3 GU, the same as that ofExample 5.

EXAMPLE 13

Example 7 was repeated except that sodium metabisulphite (0.1 g; 1.8% byweight based on the reaction mix. The colour of the product was 2.5 GU,an improvement Example 7.

EXAMPLE 14

Example 7 was repeated except that hypophosphorous acid (0.26% byweight, giving a molar ratio of acid:base of 1.2:1 was used instead ofthe phosphorous acid used in Example 7. The colour of the product was 5Gu 2 GU worse than the product of Example 7.

TABLE 1 Ex Catatyst Colour No Fatty Acid Wt % molar ratio (GU) 1 oleic2.6 1.2 3.5 1C oleic 2.3 0.8 7 2 oleic 2.3 0.9 5 3 oleic 3 1.5 5 4 oleic4.3 1.2 3.5 5 oleic 2.3 1.2 3 2C oleic 2.3 0.8 6 6 oleic 7.6 1.1 3 7oleic 8.2 1.2 3 8 oleic 8.6 1.4 4 3C oleic 6.6 0.8 6 9 oleic 9.9 1.7 54C oleic 8.2 1.2 7 10  oleic x3 5.7 1.2 5 10C  oleic x3 2.8 0.8 9 11 lauric 3.2 1.2 2-3 11C  lauric 1.6 0.8 6

EXAMPLE 15

Example 4 was repeated in a pilot scale reactor to produce about 25 kgof ester product. At the end of the esterification the colour of theproduct was 5 GU. Procedurally the ester was held in the reaction vesselunder nitrogen for an extended period at the end of which the colour hadincreased to 8 GU (because of further degradation reactions during theholding period).

EXAMPLE 16

A sample of the product of Example 15 was ethoxylated in a 19 I pilotautoclave reactor fitted with a paddle overhead agitator. Sorbitanmono-oleate (ca 1500 g; ca 3.4 mol) was charged to the reactor and wasdeaerated under vacuum at ambient temperature. The temperature wasincreased to 90° C. and sodium hydroxide catalyst added. The mix washeated further to 120° C. and water removed under vacuum. Ethylene oxidegas (ca 2990 g; ca 68 mol) was added over maintaining a constantpressure of ca 5 bar until the hydroxyl value of the product indicatednear completion of the reaction, and the system allowed to react aftercompletion of the ethylene oxide addition until the pressure fell to aconstant level. The ethoxylated product was then cooled to and vacuumstripped maintaining the temperature above 100° C., further cooled andacid added to neutralise the catalyst. This unbleached ethoxylate had acolour of 7 GU. Correcting the colour of this product to offset thehigher colour of the ester (from the holding time at elevatedtemperature) gives a value of about 3.5 to 4 GU.

The effect of bleaching this product was investigated by bleaching asample with hydrogen peroxide at 90° C. This bleached product had acolour of 5 GU. Correcting the colour of this product to offset thehigher colour of the ester (from the holding time at elevatedtemperature) gives a value of about 2 to 3 GU.

A comparative run was also carried out under the conditions of Example16 starting with commercially available sorbitan mono-oleate (Span 80 exICI Surfactants) having a colour of 8 GU. the unbleached ethoxylate hada colour of about 6 to 7 GU and the bleached ethoxylate about 5.5 GU.

What is claimed is:
 1. A method of making fatty acid esters of sorbitanwhich comprises reacting the fatty acid directly with sorbitol in thepresence of a catalyst system which comprises a phosphorus oxyacid,including a reducing phosphorus oxyacid, and an alkali or alkali earthmetal strong base in a molar ratio of acid to base of from 0.9:1 to1.7:1 and at a catalyst system concentration of from about 1.5 to about30% by weight of the sorbitol.
 2. A method as claimed in claim 1 whereinthe reducing phosphorus oxyacid is or includes phosphorous acid.
 3. Amethod as claimed in claim 1 wherein the base component of the catalystsystem is an alkali or alkali earth metal oxide, hydroxide or carbonate.4. A method as claimed in claim 3 wherein the base is sodium and/orpotassium hydroxide.
 5. A method as claimed in claim 1 wherein thecatalyst system which comprises phosphorous acid and an alkali metalhydroxide in a molar ratio of phosphorous acid to alkali metal hydroxideof from 0.9:1 to 1.7:1 and at a catalyst system concentration of fromabout 1.5 to about 30% by weight of the sorbitol.
 6. A method as claimedin claim 1 wherein in the catalyst system, the ratio of acid:base is inthe range 0.9:1 to 1.7:1.
 7. A method as claimed in claim 6 wherein theratio of acid:base is in the range 1.1:1 to 1.3:1.
 8. A method asclaimed in claim 1 wherein the catalyst system concentration is fromabout 3 to about 12% by weight based on the sorbitol.
 9. A method asclaimed in claim 8 wherein the catalyst system concentration is about 3to about 8% by weight based on the sorbitol.
 10. A method as claimed inclaim 1 wherein the reaction mixture additionally includes ametabisulphite.
 11. A method as claimed in claim 8 wherein themetabisulphite is sodium metabisulphite.
 12. A method as claimed inclaim 10 wherein the amount of metabisulphite used is from 0.1 to 10% byweight of the sorbitol.
 13. A method as claimed in claim 1 in which thereaction is carried out at a temperature in the range 170 to 230° C. 14.A method of making an alkoxylated ester of sorbitan made by the methodclaimed in claim 1 which comprises reacting the ester with an alkyleneoxide.
 15. A method as claimed in claim 14 wherein the alkylene oxide isethylene oxide.